Publications

Optical metasurfaces can manipulate electromagnetic waves in unprecedented ways at ultra-thin engineered interfaces. Specifically, in the mid-infrared (mid-IR) region, metasurfaces have enabled numerous biochemical sensing, spectroscopy, and vibrational strong coupling (VSC) applications via enhanced light-matter interactions in resonant cavities. However, mid-IR metasurfaces are usually fabricated on solid supporting substrates, which degrade resonance quality factors (Q) and hinder efficient sample access to the near-field electromagnetic hotspots. Besides, typical IR-transparent substrate materials with low refractive indices, such as CaF2, NaCl, KBr, and ZnSe, are usually either water-soluble, expensive, or not compatible with low-cost mass manufacturing processes. Here, we present novel free-standing Si-membrane mid-IR metasurfaces with strong light-trapping capabilities in accessible air voids. We employ the Brillouin zone folding technique to excite tunable, high-Q quasi-bound states in the continuum (qBIC) resonances with our highest measured Q-factor of 722. Leveraging the strong field localizations in accessible air cavities, we demonstrate VSC with multiple quantities of PMMA molecules and the qBIC modes at various detuning frequencies. Our new approach of fabricating mid-IR metasurfaces into semiconductor membranes enables scalable manufacturing of mid-IR photonic devices and provides exciting opportunities for quantum-coherent light-matter interactions, biochemical sensing, and polaritonic chemistry.

Controlling the propagation of light in the form of surface modes on miniaturized platforms is crucial for multiple applications. For dielectric multilayers that sustain Bloch surface waves at their interface to an isotropic dielectric medium, a conventional approach to manipulate them exploits shallow surface topographies fabricated on top of the truncated stack. However, such structures typically exhibit low index contrasts, making it challenging to confine, steer, and guide the Bloch surface waves. Here, we theoretically and experimentally demonstrate a device for a Bloch surface wave platform that resonantly couples light from a cavity to a straight waveguide. The structure is designed using topology optimization in a 2D geometry under the effective index approximation. In particular, the cavity–waveguide coupling efficiency of the radiation emitted by an individual source in the cavity center is optimized. The cavity is experimentally found to exhibit a narrow resonant peak that can be tuned by scaling the structure. The waveguide is shown to guide only light that resonates in the cavity. Fully three-dimensional simulations of the entire device validate the experimental observations.

Large-area imprinting stamps with nanometer-scale features are a rapidly developing area of research in plasmonics. In integrated photonic structures, surface plasmon (SPs) and surface plasmon polaritons (SPPs) are tuned by selecting both the appropriate wavelength and the angle of incidence of the excitation light. The resulting exponential fields can be studied by an optical technique such as scanning near-field optical microscopy (SNOM). Here, we report on the application of the aperture-type SNOM technique to characterize, at nanoscopic and microscopic scales, the formation of the SPPs and the beat pattern formed with the superposition of SPs and the effective component of the probing light formed in discrete metallic nanostructures of Au fabricated on imprinting stamps. We discuss a model to describe the beat pattern in terms of this superposition and demonstrate that the dominant SPs have a transverse nature. Our experiments are supported by modeling the optical response and near-field in gold nanostructures using the simulation tool Tidy3D. Our results provide a straightforward way to investigate and characterize SPPs at the nanostructure level.

The presence of long-range interactions is crucial in distinguishing between abstract complex networks and wave systems. In photonics, because electromagnetic interactions between optical elements generally decay rapidly with spatial distance, most wave phenomena are modeled with neighboring interactions, which account for only a small part of conceptually possible networks. Here, we explore the impact of substantial long-range interactions in topological photonics. We demonstrate that a crystalline structure, characterized by long-range interactions in the absence of neighboring ones, can be interpreted as an overlapped lattice. This overlap model facilitates the realization of higher values of topological invariants while maintaining bandgap width in photonic topological insulators. This breaking of topology-bandgap tradeoff enables topologically protected multichannel signal processing with broad bandwidths. Under practically accessible system parameters, the result paves the way to the extension of topological physics to network science.

Dissipative Kerr solitons from optical microresonators, commonly referred to as soliton microcombs, have been developed for a broad range of applications, including precision measurement, optical frequency synthesis, and ultra- stable microwave and millimeter wave generation, all on a chip. An important goal for microcombs is self-referencing, which requires octave-spanning bandwidths to detect and stabilize the comb carrier envelope offset frequency. Further, detection and locking of the comb spacings are often achieved using frequency division by electro-optic modulation. The thin-film lithium niobate photonic platform, with its low loss, strong second- and third-order nonlinearities, as well as large Pockels effect, is ideally suited for these tasks. However, octave-spanning soliton microcombs are challenging to demonstrate on this platform, largely complicated by strong Raman effects hindering reliable fabrication of soliton devices. Here, we demonstrate entirely connected and octave-spanning soliton microcombs on thin-film lithium niobate. With appropriate control over microresonator free spectral range and dissipation spectrum, we show that soliton-inhibiting Raman effects are suppressed, and soliton devices are fabricated with near-unity yield. Our work offers an unambiguous method for soliton generation on strongly Raman-active materials. Further, it anticipates monolithically integrated, self-referenced frequency standards in conjunction with established technologies, such as periodically poled waveguides and electro-optic modulators, on thin-film lithium niobate.

Silicon carbide (SiC)’s nonlinear optical properties and applications toquantum information have recently brought attention to its potential as anintegrated photonics platform. However, despite its many excellent materialproperties, such as large thermal conductivity, wide transparency window, andstrong optical nonlinearities, it is generally a difficult material formicrofabrication. Here, it is shown that directly bonded silicon-on-siliconcarbide can be a high-performing hybrid photonics platform that does notrequire the need to form SiC membranes or directly pattern in SiC. Theoptimized bonding method yields defect-free, uniform films with minimaloxide at the silicon–silicon–carbide interface. Ring resonators are patternedinto the silicon layer with standard, complimentary metal–oxide–semicond-uctor (CMOS) compatible (Si) fabrication and measure room-temperature,near-infrared quality factors exceeding 10 5 . The corresponding propagationloss is 5.7 dB cm−1 . The process offers a wafer-scalable pathway to theintegration of SiC photonics into CMOS devices.

Extracting photons efficiently from quantum sources, such as atoms, molecules, and quantum dots, is crucial for various nanophotonic systems used in quantum communication, sensing, and computation. To improve the performance of these systems, it is not only necessary to provide an environment that maximizes the number of optical modes, but it is also desirable to guide the extracted light toward specific directions. One way to achieve this goal is to use a large area metasurface that can steer the beam. Previous work has used small aperture devices that are fundamentally limited in their ability to achieve high directivity. This work proposes an adjoint-based topology optimization approach to design a large light extractor that can enhance the spontaneous decay rate of the embedded quantum transition and collimate the extracted photons. With the help of this approach, we present all-dielectric metasurfaces for a quantum transition emitting at λ = 600 nm. These metasurfaces achieve a broadband improvement of spontaneous emission compared to that in the vacuum, reaching a 10× enhancement at the design frequency. Furthermore, they can beam the extracted light into a narrow cone (±10°) along a desired direction that is predefined through their respective design process.

Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero-bicrystals composed of molybdenum oxide (MoO3) and hexagonal boron nitride (hBN), which feature polariton dispersion tailorable via avoided polariton mode crossings. In this work, we systematically study the polariton eigenmodes in MoO3-hBN hetero-bicrystals self-assembled on ultrasmooth gold using synchrotron infrared nanospectroscopy. We experimentally demonstrate that the spectral gap in bicrystal dispersion and corresponding regimes of negative refraction can be tuned by material layer thickness, and we quantitatively match these results with a simple analytic model. We also investigate polaritonic cavity modes and polariton propagation along “forbidden” directions in our microscale bicrystals, which arise from the finite in-plane dimension of the synthesized MoO3 micro-ribbons. Our findings shed light on the unique dispersion properties of polaritons in van der Waals heterostructures and pave the way for applications leveraging deeply sub-wavelength mid-infrared light matter interactions.

Optical microcavities confine light to wavelength-scale volumes and are a key component for manipulating and enhancing the interaction of light, vacuum states, and matter. Current microcavities are constrained to a small number of spatial mode profiles. Imaging cavities can accommodate complicated modes but require an externally preshaped input. Here, we experimentally demonstrate a visible-wavelength, metasurface-based holographic microcavity that overcomes these limitations. The micrometer-scale metasurface cavity fulfills the round-trip condition for a designed mode with a complex-shaped intensity profile and thus selectively enhances light that couples to this mode, achieving a spectral bandwidth of 0.8 nm. By imaging the intracavity mode, we show that the holographic mode changes quickly with the cavity length and that the cavity displays the desired spatial mode profile only close to the design cavity length. When a metasurface is placed on a distributed Bragg reflector and steep phase gradients are realized, the correct choice of the reflector’s top layer material can boost metasurface performance considerably. The applied forward-design method can be readily transferred to other spectral regimes and mode profiles.

Reducing geometrical complexity while preserving desired wave properties is critical for proof-of-concept studies in wave physics, as evidenced by recent efforts to realize photonic synthetic dimensions, isospectrality, and hyperbolic lattices. Laughlin’s topological pump, which elucidates quantum Hall states in cylindrical geometry with a radial magnetic field and a time-varying axial magnetic flux, is a prime example of these efforts. Here we propose a two-dimensional dynamical photonic system for the topological pumping of pseudospin modes by exploiting synthetic frequency dimensions. The system provides the independent control of pseudomagnetic fields and electromotive forces achieved by the interplay between mode-dependent and mode-independent gauge fields. To address the axial open boundaries and azimuthal periodicity of the system, we define the adjusted local Chern marker with rotating azimuthal coordinates, proving the nontrivial topology of the system. We demonstrate the adiabatic pumping for crosstalk-free frequency conversion with wave front molding. Our approach allows for reproducing Laughlin’s thought experiment at room temperature with a scalable setup.

Space-division multiplexing (SDM) with multicore fibers (MCFs) is envisioned to overcome the capacity crunch in optical fiber communications. Within these systems, the coupling optics that connect single-mode fibers (SMFs) to MCFs are key components in achieving high data transfer rates. Designing a compact and scalable coupler with low loss and crosstalk is a continuing challenge. Here, we introduce a metasurface-based free-space coupler that can be designed for any input array of SMFs to a MCF with arbitrary core layout. An inverse design technique – adjoint method – optimizes the metasurface phase profiles to maximize the overlap of the output fields to the MCF modes at each core position. As proof-of-concepts, we fabricated two types of 4-core couplers for MCFs with linear and square core arrays. The measured insertion losses were as low as 1.2 dB and the worst-case crosstalk was less than -40.1 dB in the O-band (1260-1360 nm). Owing to its foundry-compatible fabrication, this coupler design could facilitate the widespread deployment of SDM based on MCFs.

Passive daytime radiative cooling materials, capable of reducing building cooling energy by up to 60%, reflect sunlight and emit infrared radiation. The challenge lies in producing practical, durable structures. A recent publication in the Science magazine proposed the solution of using microporous glass composite with selective infrared emission and high solar reflectance, enhanced with aluminum oxide for sunlight scattering. This coating can lower temperatures by 3.5° to 4°C, even under high humidity. Remarkably, it maintains effectiveness under harsh conditions such as water, UV radiation, and extreme temperatures.

This paper discusses Anderson localization, which is the phenomenon where the propagation of diffusive waves is halted in disordered systems. Despite extensive research spanning 40 years, the localization of light in three dimensions has remained elusive, raising questions about whether it actually occurs. The text presents numerical evidence of three-dimensional localization of vector electromagnetic waves occurring within random collections of metallic spheres that overlap, which is in stark contrast to the lack of localization observed in dielectric spheres with refractive indices as high as 10 in air.

This paper presents photonic-crystal surface-emitting lasers (PCSELs) and their potential for creating large-area single-mode lasers. Scaling up PCSELs while maintaining single-mode operation is challenging, and it has impeded progress in achieving very large PCSELs. This scaling challenge arises from the diminishing quality-factor (Q) contrast between the fundamental laser mode and higher-order modes as the lateral size of the crystal increases. The text introduces the concept of bound states in the continuum (BIC), which can address this challenge.

This paper introduces a novel approach using Graph Neural Networks (GNN). This GNN architecture is designed to learn and model electromagnetic scattering and can be applied to metasurfaces of arbitrary sizes. Importantly, it considers the coupling between scatterers. As a result, this approach allows for the rapid calculation of near-fields for metasurfaces. Additionally, the approach can also be used for the inverse design of large metasurfaces, offering a versatile tool for electromagnetic field modeling and design.

In this paper, the authors propose a programmable photonic crystal cavity array and demonstrate near-complete control over the spatiotemporal properties of a 64 resonator, two-dimensional spatial light modulator with nanosecond- and femtojoule-order switching. Simultaneously operating wavelength-scale modes near the space–bandwidth and time–bandwidth limits, this work opens a new regime of programmability at the fundamental limits of multimode optical control.

In this work, the authors present a design for planar photonic topological waveguides characterized by low index contrast. Notably, they create these waveguides using polymeric materials through three-dimensional printing, allowing for rapid device fabrication. To assess the topological protection of these waveguides, they employ high-speed finite-difference time-domain simulations, particularly focusing on “omega” shaped bent topological waveguides.

Metalenses for optical beam manipulation have a significant impact in many exciting applications due their compact, planar geometry and ease of fabrication. However, the enormous physical size of metalenses relative to the optical wavelength provides a barrier to performing accurate simulations in a reasonable time frame. In principle, full-wave simulation techniques, such as the finite-difference time-domain (FDTD) method, are ideal for metalens modeling as they give an accurate picture of the device performance. However, when applied using traditional computing platforms, this approach is infeasible for large-diameter metalenses and requires hours and days to simulate even devices of modest size. To alleviate these issues, the standard approach has been to apply approximations, which typically employ simplified models of the metalens unit cells or ignore coupling between cells, leading to inaccurate predictions. In this Perspective, first, we summarize the current state of the art approaches in simulating large scale, three-dimensional metalenses. Then, we highlight that advances in computing hardware have now created a scenario where the full-wave simulation of large area metalenses is feasible within a reasonable time frame, providing significant opportunities to the field. As a demonstration, we show that a hardware-accelerated FDTD solver is capable of simulating a fully 3D, large area metalens of size 100λ × 100λ, including the focal length, in under 5 min. The application of hardware-accelerated, full-wave simulation tools to metalens simulation should have a significant impact in the metalens field and the greater photonics community. The authors wish to acknowledge the help of Lei Zheng for technical assistance. All authors have a financial interest in Flexcompute, Inc., which developed the Tidy3D solver used in this work.